In the case of restraint systems, for example in automobiles, it should be ensured for the proper operation of safety devices, such as front, knee, side and/or head airbags, that a passenger restraint system provided in a vehicle is also used and is properly locked. Otherwise, the safety devices, for example, airbag devices, could lead to injuries of the affected passenger in the case of a collision. It has been proposed to check the locking state of a latch of a seat belt that is inserted into a seat belt lock. From knowing the locking state of the seat belt lock, for example, signals can be generated in order to alert the passengers by a signal that they should put on and latch the seat belts. Ever since the introduction of airbag devices, the information on the locking state of the seat belt system has also become important for the activation or deactivation of mechanisms for inflating driver and passenger airbags or knee, side and head airbags.
Hall sensors are used as proximity switches or as sensors for non-contact determination of the state of components, which can occupy two positions. Hall sensors can include a semiconductor layer that is provided with a constant current, such as in an integrated design. The constant current is influenced by a magnetic field component perpendicular to the semiconductor layer, and the sensor yields a Hall voltage that can be evaluated, that can be tapped off and used to analyze a state, or can be employed directly as turn-on voltage. The integrated design of Hall sensors allows integrating on the Hall sensor an analysis circuit that is already suitable for analyzing the switch state.
EP-A-0 861 763 discloses a seat belt lock with an integrated prestressed Hall sensor, which detects, in a non-contact manner, the state of a locking body or an ejector for a lock latch introduced into the seat belt lock. In this case, a Hall sensor is arranged with a Hall field in the immediate vicinity of a permanent magnet. By a change in position of the locking body or the ejector, both of which elements include a ferromagnetic material, the magnetic field of the permanent magnet can be changed. As a result, the signal of the Hall sensor is also changed, and at the output of the Hall sensor, the change in state can be tapped off as a change in voltage. In an alternative variant embodiment, it is proposed to install the Hall sensor with a Hall field without a permanent magnet and for this purpose to design the locking body or the ejector as a permanent magnet. Also, in this arrangement, the change in position of the locking body or the ejector is to be detectable by a change in the Hall voltage.
In the seat belt lock according to EP-A-0 861 763, the Hall sensor is relatively sensitive compared to outside scatter fields, which can even be caused by, for example, a magnetic key attachment. The susceptibility relative to outside scatter fields can also be magnified in that the signal changes, because of the comparatively short distances that are covered by the locking body or the ejector when opening or closing the seat belt locking, are relatively small. Also, the seat belt lock variants without prestressed Hall sensors, in which either the locking body or the ejector are designed as permanent magnets, have proven to be not very practicable. The signal changes that can be achieved are also relatively small here. By the vibrations of the locking body and the ejector when the seat belt lock is opened and closed, demagnetization can result over time in the case of known magnets. This ultimately can lead to the Hall sensor no longer detecting the changes in state of the seat belt lock.
To reduce fuel consumption, major efforts are being undertaken to reduce the weight of inserts and attachments of motor vehicles. To this end, inserts, such as, for example, seat belt locks of restraint systems, are also made smaller. Because of the smaller space that is available, the devices for detection of the locking state of the seat belt locks should also be modified. With respect to the desired robustness of the devices, and to ensure, for example, up to 100,000 cycles of operation, such modifications may not be trivial.